Pseudo‐Bifurcation of Common On‐Axes Poisson's Ratios in a Class of 2D Model Metamaterials

This article identifies and analyzes a new class of metamaterials that exhibit a previously unreported pseudo‐bifurcation of on‐axes Poisson's ratios. The metamaterials consist of rotating rhombic units, each composed of four triangular or trapezoidal subunits, which undergo either a fragmentation mechanism, whereby the subunits rotate relative to one another while preserving overall unit orientation, or a reconstitution mechanism in which all subunits rotate as a rigid body. Owing to their kinematic architecture, both metamaterials share common on‐axes Poisson's ratios at two critical configurations: (i) in the reconstituted state and (ii) at the instant just prior to fragmentation. Beyond this point, under finite rotation of the subunits, the two on‐axes Poisson's ratios bifurcate. These conditions correspond to unique geometric configurations that delimit the transition between common and bifurcated Poisson's ratios. The resulting metamaterials combine Poisson's ratio sign‐switching, discontinuity, constancy under reconstitution, and pseudo‐bifurcation under fragmentation—features not simultaneously present in earlier fragmentation‐reconstitution (FR) metamaterials.